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Ocean Surface Topography from Space
The Harvest experiment

Figure 1


B. Haines (NASA/JPL, USA),
Y. Bar-Sever (NASA/JPL, USA),
G. Born (University of Colorado, USA),
E. Christensen (NASA/JPL, USA),

Bruce Haines
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109 - USA



Every ten days, the Jason-1 satellite will directly overfly an oil platform (Harvest) off the
coast of central California. Data from specialized instruments on the platform will be used to
verify the accuracy of the Jason-1 sea-level measurements.


The Arguello Inc. Harvest Oil Platform (figure 1) is located about 10 km off the coast of central
California near Point Conception. An impressive structure, the platform is attached to the sea
floor and sits in about 200 m of water near the western entrance to the Santa Barbara Channel.
Conditions at Harvest are typical of the open ocean and the seas can be quite heavy. Ocean swell
and wind waves average about 2 m, though waves over 7 m have been experienced during powerful
winter storms. Prevailing winds are from the northwest and average about 6 m/s (15 mph). The
platform is served by helicopters from the Santa Maria, California, airport, and is regularly
visited by supply boats. Operational since 1991, Harvest has produced over 44 million barrels
of oil (as of July, 1997).

In addition to its primary mission to drill for oil, Harvest is a "calibration site" for the
Jason-1 (2001 launch) and TOPEX/POSEIDON (1992-) missions. As such, The platform is an important
international resource for the study of sea level from space. A consideration in designing the
TOPEX/POSEIDON (T/P) orbit was a requirement that the satellite pass directly over Harvest on a
regular basis. Traveling in excess of 7 km/s at an altitude of 1335 km, T/P flies over the
platform every 10 days en-route to tracing out its global pattern of sea-level measurements.
Observations recorded with instruments attached to the platform are used to verify the sea-level
readings from T/P taken at the instant of the overflight. After its own launch this summer,
Jason-1 will be placed in the same orbit, bringing it over Harvest in formation flight with T/P.

Why is Harvest such a good calibration site? For one, it is located sufficiently far offshore
so that the area illuminated by the altimeter's radar pulse is covered entirely by ocean when
the satellite is directly overhead. At the same time, the platform itself is small enough so
that it cannot influence the reflected radar signal. The platform's location in the open
ocean also implies that the altimeter missions are monitored in conditions under which their
measurements systems are designed to best operate. Finally, the platform is located in
proximity to important tracking stations in California and the western U.S., data from which
contribute to measuring the positions of the satellite and platform through space-based
surveying techniques.

The fundamentals of the calibration technique are well-established from previous missions.
Attached to the platform are "tide gauges" that continuously measure variations in the
sea level relative to the platform. These variations may be due to the effects of
fluctuating ocean currents and atmospheric pressure in addition to astronomical tides.
Any sinking or rising of the platform structure itself will also affect the tide-gauge
measurements. As oil is pumped from the underlying deposit, for example, the sea floor
supporting the platform structure subsides by almost 1 cm each year. This is measured
using data from a global positioning system (GPS) receiver on the platform. Also computed
with the GPS data is the absolute height of the platform relative to Earth's center.

Combining the tide gauge and GPS results gives the local sea-surface height (SSH),
relative to Earth's center, the same quantity measured by the radar altimeter when it
flies over the platform. By looking at the mismatch of the two SSH readings (Jason-1
vs. Harvest) at the overflight times, the errors in the respective measurement systems
will be exposed. Errors in the platform measurements are minimized by using redundant
systems and through careful monitoring and routine maintenance of the Harvest instruments.
Drawing on our experience with T/P, we expect the agreement between the instantaneous
sea-level readings (Jason-1 vs. Harvest) for a typical overflight to be 2-3 cm or better.
A multi-year time series of these overflight comparisons beginning with the launch of
Jason-1 will serve as a vital performance record for the mission.

The value of the Harvest experiment was amply demonstrated with results from the T/P
mission. Shortly after the satellite launched in 1992, results from Harvest showed that
the TOPEX sea-level measurements were too high by almost 15 cm [Christensen et al., 1994].
The bias is now recognized as a consequence of an error in the software used to produce
the TOPEX data for the mission scientists [Nerem et al., 1997]. As T/P continues its
successful run, attention has turned to improving estimates of measurement-system
stability in order to verify the emerging record of global sea-level change [see also
Mitchum, 1998].

The T/P experience has proven to be excellent preparation for Jason-1. We expect even
greater success for Jason-1, owing to improvements in both the platform and satellite
systems. In addition, during the first few months of its mission, Jason-1 will pass over
Harvest in formation flight with T/P. Because the ocean scene at Harvest cannot change
appreciably in the few seconds separating the T/P and Jason-1 overflights, many sources
of measurement uncertainty will cancel. This cancellation will significantly benefit
cross-calibration of data from the two satellites. Jason-1 will also benefit from the
ongoing minimization of systematic errors in the platform measurement system. Particular
emphasis is being placed on monitoring the platform vertical position and velocity with
GPS at the 1-cm and 1-mm/yr levels, respectively. We will also exploit the thorough
instrumentation at Harvest in order to lend insight on the source of the potential
errors in the Jason-1 data (e.g., atmospheric delays, range anomalies). Throughout
the Jason-1 mission, we will work closely with other Jason-1 investigators overseeing
additional regional calibration sites, as well those performing drift calibrations using
global tide gauges, to reach a consensus on the application of calibration corrections to
the global mean sea level record.


Christensen E.J. et al., l994: Calibration of TOPEX/POSEIDON at Platform Harvest. J. Geophys. Res., 99, C12, 24,465-24,485

Mitchum G., 1998: Monitoring the stability of satellite altimeters with tide gauges. J. Atmos. and Oceanic Tech., 15(3), 721-740.

Nerem R.S., et al., 1997: Improved determination of global mean sea level variations using TOPEX/POSEIDON altimeter data. Geophys. Res. Lett., 24(11), 1331-1334.

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